1. Field of the Invention
The present invention relates to turbofan engines commonly used in aircraft travelling at subsonic speeds, and more particularly to a mechanism for reducing fan rotational speed.
2. Description of the Related Arts
Turbofan engines have been widely used for aircraft travelling at subsonic speeds. In turbofan engines, propulsive force is gained by an exhaust stream, i.e. a jet, of a core engine in the form of a gas turbine engine and a fan driven by shaft power of the core engine.
In such a turbofan engine, the specific fuel consumption can be reduced by increasing the bypass ratio, that is, the ratio of the flow rate of the air directed downstream by the fan but bypassing the core engine to be discharged to the flow rate of air taken into the core engine. That is, higher efficiency can be obtained by increasing the propulsive force by the fan relative to the propulsive force by the jet.
A greater bypass ratio can be obtained by increasing the outer diameter of a fan. However, as the outer diameter of the fan increases, the peripheral speed of the fan increases and shock waves are produced when the speed approaches that of sound. This results in a lowering of the propeller efficiency of the fan, which in turn leads to a decline in overall efficiency. Therefore, rotational speed of the fan must be decreased in order to increase the bypass ratio. If the rotational speed of the fan is simply lowered, the rotational speed of the turbine in the core engine which directly drives the fan is also lowered, resulting in a decrease in turbine efficiency. Consequently, thermal efficiency of the core engine is also declined, thereby decreasing the overall efficiency.
A technique has been proposed to use a gear mechanism to reduce the rotational speed of the rotation shaft of the core engine and rotate the fan at a reduced speed. This technique makes it possible to obtain appropriate rotational speeds of both fan and turbine. For aircraft application, such speed reducing mechanism is required to be extremely lightweight and highly reliable.
Material for the gear mechanism is relatively heavy because of durability requirements. If its weight is reduced, rigidity of the gear mechanism will be diminished, giving rise to great deflection of components. Such deflection makes it extremely difficult to accurately obtain the engaging positions of the gear teeth, and therefore it is almost impossible to accurately know the dynamic stress at the components of the gear mechanism. Thus, the gear mechanisms currently available do not simultaneously meet requirements for reduction in weight and for high reliability.
Further, the fan and the turbine of the core engine produce thrust in the opposite directions. Therefore, if a gear mechanism as described above is provided between the fan and the turbine, these thrusts are not cancelled and a thrust bearing with great capacity is necessary, which further increases the total weight. If such a gear mechanism is not provided, the turbine and the fan are provided on a common shaft, substantially canceling the thrust powers, and only a small thrust bearing is required.
From the reasons above, application of the above gear mechanism to aircraft is limited to small engines; its application to large-scale engines has yet to be practically implemented. In addition, no appropriate speed reducing mechanisms other than such a gear mechanism have either been implemented or proposed.